Advances in the development and improvements of the luminous flux of light-emitting devices such as solid-state semiconductor and organic light-emitting diodes (LEDs) have made these devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting. Light-emitting diodes are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps. Also, with the increasing selection of LED wavelengths to choose from, white light and colour changing luminaires are becoming more popular. As such, there is an ever present need for improved control over the colour of light output by these luminaires.
In general, a white light or colour changing luminaire comprises different, single wavelength LEDs, such as red (R), green (G), blue (B) and optionally amber (A) LEDs. The colour of the light output by the luminaire can be changed and/or optimised by altering the relative intensities of these LEDs.
One difficulty generally associated with white light or colour changing luminaires, however, is that the output of each LED changes differently with changes in ambient and/or operating temperature. If the ratio of average driving currents in the LEDs is not changed to accommodate this situation, the combined output colour of the luminaire may change by an often perceivable, and generally undesired amount. Other factors that may affect the output colour may include ageing of the LEDs and the luminaire's output intensity.
Consequently, in order to achieve a stable output colour, that is an output colour that remains substantially the same despite ambient and/or device temperature changes, ageing of the LEDs, and/or variations in overall output intensity, it may be necessary to monitor the chromaticity of the combined LED outputs and provide feedback to a controller thereof, which controls the LEDs to substantially maintain a desired output colour. Such feedback control may also be applicable to substantially maintain a desired luminaire output luminosity, radiometric power, and the like, as well as maintain and/or optimise a luminaire's colour quality and/or colour rendering efficiency.
The following provide some examples of light sources having a feedback control system. For instance, in U.S. Pat. No. 6,964,500 a liquid crystal display apparatus is described as having red, green and blue LEDs directed and coupled into a light guide plate for evenly guiding and diffusing the LED light over a large output surface area of the plate via a diffusion portion having a printed microdot pattern. An optical sensor, coupled to a side surface of the light guide plate via a light-shielding member, is also provided for detecting light exiting from the light guide plate through this side surface, and controlling, based on this detected light, the luminance of the LEDs.
In U.S. Pat. No. 6,753,661, an apparatus and method for backlighting an electronic display is described as using LEDs and a feedback control microprocessor for controlling a luminosity, radiometric power, and colour levels of the apparatus, thereby maintaining a substantially constant white backlight level. A similar backlighting application is also described in the Philips Lumileds Lighting Company document entitled “LED Backlighting from Revolution to Reality”, published Jun. 7, 2004 (http://www.lumileds.com/pdfs/tp39_us_fpd—2004.pdf), whereas the OSRAM Application Note on Light Guides, published Apr. 25, 2002 (http://catalog.osramos.com/media/_en/Graphics/00026784—0.pdf) provides some general guidelines regarding planar light guides as they relate to such backlighting applications.
Also, a luminaire comprising a feedback control system is described in U.S. Pat. No. 6,741,351, wherein a white light emitting LED luminaire combines an array of red, green and blue LEDs with a feedback arrangement configured to maintain a desired colour balance. The feedback arrangement includes photodiodes positioned to intercept reflected light from a partially reflecting element placed in the path of the combined output of the LED array. Alternatively, each LED has an associated photodiode, either incorporated into the LED package or in a separate package, to provide similar results.
In general, the above and other such currently available light sources have various drawbacks, namely with regards to the optical coupling of LED outputs to the sensor(s) of a feedback control system. For instance, unwanted ambient light may be incident on the sensor and cause errors in the feedback signal, light reaching the sensor(s) may not be sufficient to provide an adequate feedback signal, the respective contributions of the LEDs may not be sufficiently balanced and/or the sensor(s) may obscure part of the light source's output.
Consequently, there is a need for a light source that overcomes some of the drawbacks of known light sources.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.